For an enlarged projection of images on a screen by means of a light valve projector, an optical image is put on a light valve and bundled light is sent through said light valve. Said light then passes through an appropriate projection lens system, which focuses an image on the screen. An appropriate light valve for such a projection system is a liquid crystal display, commonly referred to as an "LCD".
Two kinds of liquid crystal light valve projectors exist: OFF-axis projectors and ON-axis projectors.
FIG. 1 shows the optical path in an OFF-axis projection system. Said projection system comprises an OFF-axis projector 1 and a screen 2. The OFF-axis projector 1 comprises an illumination system 3, a field lens 4, a polariser 5, an LCD 6, an analyser 7 and a projection lens 8. The illumination system 3 comprises a lamp, a reflector, and an integrating system which converts the circular and non-uniform light distribution coming from the reflector into a rectangular and uniform illumination of the LCD 6. The field lens 4 images the illumination system into the entrance pupil of the projection lens 8. In this way, it ensures that the light with which the LCD 6 is illuminated is directed into this entrance pupil of the projection lens 8. The projection lens 8 images the illuminated LCD 6 on the screen 2. The line through the optical centres of the illumination system 3, the field lens 4 and the projection lens 8 is further called the optical axis 9 of the projector 1.
In case the OFF-axis projector 1 is a multi-light valve projector (for example a projector with three light valves), the optical path is more complex, because by means of a set of dichroic mirrors the light is splitted into 3 colours (red, green and blue), and after modulation of the light, these colours are recombined again. The OFF-axis projector then comprises an illumination system, three field lenses, three sets consisting of a polariser, an LCD and an analyser, and a projection lens. A line through the optical centres of the illumination system, one of the field lenses and the projection lens is an optical axis of the projector. The schematic description of the illumination and imaging system shown in FIG. 1 is not altered by these colour-splitting mirrors. In what follows the state of the art is described as if there were only one illumination system, one field lens, one set consisting of a polariser, an LCD and an analyser, and one projection lens.
The direction of the optical axis 9 of the projector 1 is different from the direction of the optical axis 10 of the projection lens 8. Both optical axes 9 and 10 include an OFF-axis angle .alpha. as shown in FIG. 1. This angle .alpha. can have any positive value, but in the most common OFF-axis configuration, the projector 1 is set up in such a way that the optical axis 10 of the projection lens 8 intersects with the bottom of the image of the screen 2.
If the OFF-axis angle .alpha. is zero, or in other words, if the direction of the optical axis 9 of the projector 1 is equal to the direction of optical axis 10 of the projection lens 8, the projector is called an ON-axis projector. FIG. 10a shows the minimum configuration of an ON-axis liquid crystal light valve projection system. It comprises an ON-axis projector 11 and a screen 2. The ON-axis projector 11 comprises the same parts as the OFF-axis projector 1 described hereinabove. Said same parts have been denoted by means of the same reference numerals.
FIG. 2 shows an image source of as well OFF-axis and ON-axis liquid crystal light valve projectors 1 and 11. Said image source comprises a polariser 5, an LCD 6 and an analyser 7. Although represented in FIG. 2 as three separate parts, two of the parts or the three parts can form one unit. The polariser 5 and the analyser 7 are both light polarising elements and have in their planes each an absorbing direction and a non-absorbing direction, the absorbing direction being perpendicular to the non-absorbing direction.
The LCD 6 comprises, with reference to the light direction, a front glass plate 15, a back glass plate 16 and sealed in between both glass plates 15, 16 a twisted nematic liquid crystal layer. The sides of the glass plates 15 and 16 not touching the twisted nematic liquid crystal layer, each have a layer with transparent image forming pixel electrodes. The twisted nematic LCD (TN-LCD) may be addressed with an active matrix. In such an active matrix TN-LCD, a switching device, such as a thin film transistor or a number of thin film diodes, is integrated on each pixel.
In FIG. 2, the non-absorbing direction 17 of the polariser 5 is perpendicular to the rubbing direction 18 of the front glass plate 15 of the LCD 6. Said rubbing direction 18 defines the preferred direction of the liquid crystal molecules at the interface with the front glass plate 15. The non-absorbing direction 19 of the analyser 7 is perpendicular to the rubbing direction 20 of the back glass plate 16. In a twisted nematic liquid crystal display, the rubbing directions at the opposite glass plates are perpendicular, and therefore the molecules form a 90 degrees twisted helix in the bulk as shown in FIG. 3a and FIG. 3b. FIG. 3a shows the distribution of the molecules 21 in a twisted nematic LCD between the front glass plate 15 and the rear glass plate 16 in the bright state. FIG. 3b shows the corresponding distribution in the dark state.
To obtain an optimum black level in an image projected by a projection system containing a polariser, an LCD and an analyser, the non-absorbing directions of these three elements must be correctly matched. Therefor, the analyser or the polariser may be installed rotatable around their normal.
The contrast of the image obtained with an LCD as light valve depends strongly on the angle of incidence of the light entering the LCD. It is by consequence LCD area and, by consequence, screen area dependent, what is explained by the following.
FIG. 4 shows the typical viewing angle characteristics of a twisted nematic LCD. The graphs are lines of equal contrast between the bright and dark state at the two corresponding driving voltages, with relative values between 3 and 300. The vertical axis shows the vertical angle of incidence between -40.degree. and 40.degree.. It presents the component of the light ray within the plane perpendicular to both the plane of the LCD and the horizontal image scanning direction of the LCD. The horizontal axis gives angles of incidence between -40.degree. and 40.degree. and is the component of the light ray within the plane perpendicular to the plane of the LCD and the vertical image scanning direction of the LCD.
FIG. 4 thus demonstrates the dependence of the contrast in function of the incident angle of the light which enters the LCD. The contrast changes much more in the vertical direction than in the horizontal direction of the screen. Moreover, the maximum contrast is not obtained for light rays with normal incidence but for light rays having a small incident angle in the vertical direction. This angle corresponds with the angle .alpha..sub.v shown on FIG. 3b. The angle .alpha..sub.v corresponds very well with the remaining tilt at dark state of the molecules in the twisted nematic liquid crystal layer.
FIG. 1 and FIG. 10a show that light rays enter the LCD under different angles. As a consequence of the phenomena shown in FIG. 4, without further measures being taken, an image projected by a projector using as light valve a twisted nematic LCD, will be non-uniform, which is differently contrasted in vertical direction.
FIG. 5 shows in another way how the LCD light transmission (vertical axis) depends on the angle of incidence (different graphs) and on the LCD drive voltage (horizontal axis). For angles of light incidence larger than 0.degree., the light transmission graphs show a minimum. This minimum determines the maximum obtainable contrast at that angle of incidence, as for higher drive voltages the contrast is negative. For larger positive angles of incidence, the minimum transmission value is higher and so the maximum obtainable contrast is lower. For smaller positive angles of incidence, the location of the minimum on the graph is shifted to a higher drive voltage and to a lower transmission value or higher obtainable contrast. For zero and negative angles of incidence of light, there is no minimum of transmission.
Another representation of the contrast dependence of the angle of incidence, also including the behaviour of the LCD in function of its drive voltage, is given by the graphs "0" in figures FIGS. 6a, 6b and 6c. The normalised intensity on the screen is displayed on these figures in function of the LCD drive voltage for a projection system as illustrated by FIG. 1. FIG. 6a shows the intensity measured at the top of the display screen; FIG. 6b shows the intensity in the middle of the screen; FIG. 6c shows the intensity measured at the bottom of the screen.
FIG. 6a shows that in areas of the image where the angle of incidence of light is larger, the intensity of the projected image has a minimum value for a specific drive voltage of the LCD. This is the case at the top of the image in a projection system as illustrated in FIG. 1. For drive voltages above the voltage of minimum intensity, the contrast is even negative. If otherwise the drive voltage should be equal all over the LCD, such an intensity minimum will appear on the screen at a specific angle of incidence and be visible as horizontal darker bar.
FIG. 6b and FIG. 6c show that in areas of the image where the angle of incidence is smaller, such a minimum of intensity (or in other words maximum of contrast) does not occur. There, the limitation is determined by the maximum LCD drive voltage allowed.
For the above reasons, in practice the LCD drive voltages are chosen so as to get a uniform black level or minimum intensity all over the screen. The limit is thereby determined by what can be obtained in the area of larger angles of incidence, and so does not result in the best contrast possible in other areas of the screen.
In order to obtain a better contrast and better uniformity of contrast without loss of other picture quality, various methods of contrast enhancement and apparatus with improved contrast are known.
Various apparatus have additional physical items with reference to the minimum projector set-up as described in FIG. 1.
In U.S. Pat. No. 5,576,854, a quarter-wave compensator plate is disposed between the polariser and the liquid crystal light. This solution is only suitable for the reflective set-up described in the patent and is not applicable for projection with TN-LCDs.
In U.S. Pat. No. 5,371,559, a stop is disposed inside the projection lens, forming an aperture of the lens assembly decentered from the optical axis of the lenses of the projection lens assembly in the vertical scanning direction of the liquid crystal panel. The blocking of the light rays at the entrance pupil of the lens reduces the efficiency of the projection system, resulting in a lower light output of the projector.
In U.S. Pat. No. 5,375,006, contrast improvement in a projector using TN-LCDs is obtained by using (uniaxial) birefringent films. In some of the claims, the films are even inclined with respect to the optical axis. This solution however complicates the complete stack of elements of the LCD light-modulating device, because two new optical surfaces are introduced. This leads to light loss due to extra reflections. If the films are optically coupled to minimise this light loss, then the LCD itself becomes thicker, and more difficult to cool. Moreover, these thin birefringent films are also sensitive to temperature changes and mechanical stress caused by the temperature differences in a high light-output projector.